Hitherto, there are various publicly known technologies for protecting a rider by mounting an airbag apparatus to a motorbike. For example, in a motorcycle, a technology in which an airbag stored in a case mounted to a vehicle body frame is deployed and inflated by inflation gas, and the rider is restrained thereby in case of a front collision is publicly known (for example, see Japanese Unexamined Patent Application Publication No. 2002-137777). In this technology, a possibility to provide a wide area to be protected by the airbag is proposed. However, in the case where the airbag apparatus is mounted to a vehicle body which is opened in all directions such as a motorbike, a further effective technology for ensuring restraint of the rider by the airbag is in strong demand.